A program is underway to study the stereoselectivity requirements of various steroid hydrogenases and dehydrogenases. The studies are being conducted using microorganisms which have the properties to hydrogenate carbon-carbon double bonds situated at positions 1 and 4 in the steroid skeleton. Other experiments are presently in progress to investigate the mechanism of action of steroid dehydrogenases, capable of forming double bonds at these positions. The structural reqirements for these enzymic reactions are being determined using deuterium labeling, to discover whether a cis or trans hydrogen addition (or elimination) is taking place. Similar studies are contemplated for double bonds at other positions of the steroid nucleus. This information can be useful in designing new drugs that are inactivated by such enzymes.